An RNA virus population does not consist of a single genotype;rather, it is an ensemble of related sequences, termed quasispecies. High mutation rates of RNA viral replication create a 'cloud'of potentially beneficial mutations at the population level, which afford the viral quasispecies a greater probability to evolve and adapt to new environments and challenges during infection. Using poliovirus as our model we developed strategies to increase or reduce the mutation rate of the viral polymerase thus changing the levels of genomic diversity in the viral population. In infected animals, reducing or increasing viral diversity leads to loss of neurotropism and an attenuated pathogenic phenotype. These findings suggest that quasispecies diversity is finely tuned to ensure evolutionary survival of the virus and is a biological determinant for the outcome of poliovirus infection. The long-term goal of this proposal is to examine the significance of genome diversity for virus pathogenesis. Our research will have important consequences for the understanding of viral pathogenesis and for the development of live attenuated viral vaccines. Specifically we propose to: (1) characterize the relationship between polymerase fidelity and the generation of viral quasispecies;(2) determine the relationship between viral quasispecies diversity, fitness and adaptability;(3) characterize the relationship between quasispecies diversity and pathogenesis in vivo, in infected animals, (4) explore the possibility of developing live- attenuated virus vaccines by altering quasispecies diversity.